This invention relates to profiling the intensity distribution of optical beams.
Various schemes have been developed for controlling the amount of optical energy received by a target surface to achieve e.g., a desired ablation profile, or a desired profile of optical exposure.
For example, many photorefractive keratectomy (PRK) procedures require the delivery of a precise dose of optical energy to the cornea of a patient's eye suffering from e.g., myopia or hyperopia to remove corneal tissue in a controlled fashion to shape the surface of the cornea to change the radius of curvature, or refractive power, of the patient's eye.
The cornea comprises transparent avascular tissue that forms the anterior portion of the eye. The cornea functions as both a protective membrane and a "window" through which light passes as it proceeds to the retina. The transparency of the cornea is due to its uniform structure, avascularity, and deturgescence, which is the state of relative hydration of the corneal tissue. The average adult cornea is about 0.65 mm thick at the periphery, and about 0.54 mm thick in the center. From anterior to posterior, the cornea has the following five distinct layers: the epithelium, Bowman's membrane, the stroma, Descemet's membrane, and the endothelium.
A major proportion of the refractive power of the eye is determined by the curvature of the anterior surface of the cornea, so that changing the shape of the cornea offers a way to significantly reduce or eliminate a refractive error in the eye.
The general technique for shaping the cornea of a patient's eye involves removing the epithelial layer, and then shaping the underlying Bowman's and stroma layers, either surgically, or by using photoablation with e.g., ultraviolet radiation from an excimer laser or infrared laser radiation from an infrared laser operating at a wavelength of about 2.6-3.2 .mu.m.
In radial keratotomy, a set of radial incisions are made in the stroma to change the eye curvature, as described in Schneider et al. U.S. Pat. No. 4,648,400.
Another technique, described in Muller, U.S. Pat. No. 4,856,513 (assigned to the present assignee), uses a laser and an erodible mask with a predefined profile of resistance to erosion by laser radiation disposed between the laser and the corneal surface. A portion of the laser radiation is absorbed by the mask, while another portion is transmitted to the corneal surface in accordance with the mask profile, thereby selectively photoablating the corneal surface into a desired shape.
In yet another technique, described in Marshall et al., U.S. Pat. No. 4,941,093 (assigned to the present assignee), the shape and size of the area of the corneal surface irradiated by laser energy is selected and controlled with an adjustable aperture or lens so that some areas of the corneal surface become more ablated than other areas, whereby a desired corneal shape can be achieved.
Alternatively, the cornea can be shaped by controllably scanning laser beams across the corneal surface, which have small spot sizes relative to the size of the ablation area.
Many conventional photorefractive keratectomy (PRK) procedures employ laser radiation from an excimer laser. However, a laser pulse from an excimer laser generally has a nonuniform intensity profile, and its beam diverges more than most lasers.
A typical excimer laser, operating at 193 nm, has an intensity profile that is about 8 mm by 24 mm with a gaussian distribution across the short dimension and .+-.10% intensity variation across the long dimension, and has an intrinsic divergence of about 1 milli-radian.
The ultimate commercial success of PRK will be determined by the predictability, stability, and safety of the procedure.
A simple, reproducible laser ablation technique could complement or replace the above-mentioned techniques and could further advance the field of corneal shaping.